This article demonstrates the feasibility of efficient energy harvesting for sub-Watt, small-scale systems combined with durable energy storage in the form of supercapacitors, which are expected to last for 20 years at non-trivial power levels.
Previous maximum power point tracking systems were designed for large systems that optimize for conversion efficiency, but for small wireless sensor nodes, it is critical to maximize absolute converted power by minimizing MPPT overhead.
Described here is an efficient charging method for a supercapacitor-operated, solar-powered wireless sensornode called Everlast. Unlike traditional wireless sensors that store energy in batteries, Everlast’s use of supercapacitors enables the system to operate for an estimated lifetime of 20 years without anymaintenance.
The novelty of this system lies in the feed-forward, pulse frequency modulated converter and open-circuit solar voltage method for maximum power point tracking (MPPT), enabling the solar cell to efficiently charge the supercapacitorand power the node.
Experimental results show that by its low-complexity MPPT, Everlast can achieve over 89% conversion efficiency with lower power overhead than the state-of-the-art bytwo orders of magnitude, while enabling charging a supercapacitor up to 400% faster than direct charging.
Our system also charges the samesupercapacitor to 5 V 25% faster, to 2.5 V 100% faster, and toeven higher voltage 300% to 400% faster than direct charging. Everlast also passed multi-day stress tests while performing a variety of wireless sensing tasks under different weather conditions.
Future work includes fine-tuning of the present designto achieve higher efficiencies at lower solar power levels, further reducing the sleep mode power consumption, and furtherscaling onto an SoC to operate as a micro-Watt sensor node.
To read this external content in full download the complete paper from the author archive online.